Which kind of graphite: flaky graphite, graphite powder, exfoliated graphite, ...? The percolation threshold will be drastically different, depending on the morphology of the filler. Flakes also tend to orientate mutually, and hence to lead to strongly anisotropic composites. So, depending on the direction of sollicitation, the results will be totally different.

Alain

Several empirical models have been proposed for the prediction of mechanical properties of polymer composites depending on "a lot of parameters". These parameters can be basically the intrinsic properties polymer and filler also include the nature, size, shape, load (%), distribution... of filler as well as the polymer/filler interaction...    

As said Alain, the tensile strength would depend on the parameters of  graphite as well.